1. Field of the Invention
The present invention relates to a tone controller and a receiver having the tone controller circuit, and more particularly, to a tone controller circuit for attenuating the noise contained in the signal which a receiver generates from a weak electric field, and also a receiver having this tone controller.
2. Description of the Related Art
Generally, the amplitude of the signal a receiver has generated by demodulating FM or AM waves is proportional to the intensity of the electric field of the FM or AM waves. Hence, the lower the intensity of the electric field, the smaller the amplitude the signal has. Further, the smaller amplitude of the signal, the smaller gain and the lower signal-to-noise (S/N) ratio the signal will have. In particular, a signal the receiver has generated from FM waves containing noise components of relatively high frequencies, i.e., so-called triangular noise-waves, has a very low S/N ratio. In order to improve the S/N ratio of the signal, a high-cut controller is used in a receiver. This controller attenuates the high-frequency noise components contained in the signal generated by demodulating the waves of a weak electric field, to the degree inversely proportional to the amplitude of the signal.
More specifically, in such a prior-art receiver, an intermediate-frequency (IF) amplifier performs limited amplification on a signal supplied from the antenna via a front-end, a demodulator demodulates the signal output by the IF amplifier into an audio signal, and a high-cut controller (for example, TA8130Z manufactured by Toshiba) attenuates the high-frequency component of the audio signal. Part of the signal output by the IF amplifier is input to an amplitude detector. The detector outputs a control signal at a level proportional to the amplitude of the input signal. The control signal is adjusted by a semi-fixed resistor, so as to be input to the high-cut controller through a sensitivity adjuster. If the output signal of the demodulator has been generated from weak waves, its high-frequency component is greatly attenuated by means of the high-cut controller. By contrast, if the output signal of the demodulator has been generated from strong waves, its high-frequency component is not attenuated by the high-cut controller.
To attenuate the noise component of the output signal of the demodulator, which has been generated from a low-intensity electric field, the high-frequency component of the signal is attenuated in reverse proportion to the voltage of this signal, thereby to increase the S/N ratio of the signal.
As far as the voltage of the signal output by the demodulator is as low as the voltage required for driving the high-cut controller, the high-cut controller can sufficiently attenuate the high-frequency component (demodulation frequency fm=10KHz), thus sufficiently reducing the noise component. The noise reproduced from the audio signal output by the receiver is indeed far less prominent to the listener than the noise reproduced from the audio signal generated by a receiver without a high-cut controller, but the sound reproduced from the same audio signal is less clear than in the case where the receiver has no high-cut controllers to attenuate the high-frequency component of the signal output by the demodulator. This is because the main components of the audio signal are of low frequency and intermediate frequency.